1. Field of the Invention
The present invention relates to a cellulose acylate film, a process for producing it, and a polarizing plate and a liquid crystal display device having it.
2. Description of the Related Art
Liquid crystal display devices have been used for more and more various applications such as monitors of personal computers, mobile phones and televisions since they have various advantages such as small voltage and power consumption, and downsizing and thinning. Various modes of liquid crystal display devices depending on the state of liquid crystal molecules in the liquid crystal cell have been provided. Previously, a TN-mode employing a liquid crystal cell in which liquid crystal is twisted at about 90 degrees between the upper and lower substrates has been major. In general, a liquid crystal display device comprises a liquid crystal cell, an optically-compensatory film(s), and a polarizing element(s). The optically-compensatory film serves to cancel image coloration and to widen a viewing angle, including a stretched birefringent film and a film produced by coating a transparent film with a liquid crystal. For example, a technique of applying an optically-compensatory film prepared by applying a discotic liquid crystal to a triacetyl cellulose film and aligning and fixing it thereon, to a TN-mode liquid crystal cell to thereby widen a viewing angle has been proposed. However, for a liquid crystal display device for TVs that are expected to be watched at various angles on a large-size panel, the requirement in point of the viewing angle dependence thereof is severe, and still could not be on a satisfactory level even though the above-mentioned technique is applied thereto. Accordingly, others than TN-mode liquid crystal display devices, such as IPS (in-plane switching) mode, OCB (optically compensatory bend) mode and VA (vertically aligned) mode devices are now under investigations. In particular, VA-mode devices have a high contrast ratio and the production yield thereof is relatively high, and therefore they are now being in the mainstream of liquid crystal display devices for TV.
Cellulose acylate films exhibit high optical isotropy, that is, low retardation, compared with other polymer films. So, usually, they have been utilized in various applications such as protective films of polarizing plates, which are required to be optically isotropic.
U.S. Pat. No. 6,503,581B2 discloses a fatty acid cellulose ester film having an acetyl group and a propionyl group, wherein a sum of degree of acetyl substitution (DSac) and degree of propionyl substitution (DSpr) of the fatty acid cellulose ester of the fatty acid cellulose ester film is 2.8 or less, and a retardation value (Rt value) in the thickness direction is 60 to 300 nm; and also discloses, as an embodiment thereof, a fatty acid cellulose ester film having DSac of from 1.5 to 2.3, DSpr of from 0.5 to 1.2, and Re of equal to or smaller than 30 nm.
JP-A 2007-3767 discloses a polarizing plate having predetermined properties, and also discloses, as an embodiment thereof, a polarizing plate having a cellulose ester film, of which retardation in-plane is from 40 nm to 50 nm and retardation along the thickness direction is from 110 nm to 140 nm, containing cellulose acetate propionate having the total degree of substitution with acyl of from 2.10 to 2.65 and the degree of substitution with propionate of from 0.7 to 1.5.
WO2007/125764A1 discloses a retardation film containing a predetermined ester compound and a cellulose derivative; also discloses, as an embodiment thereof, a retardation film having retardation in plane, Ro, of from 20 nm to 80 nm, retardation along the thickness direction, Rth, of from 100 nm to 250 nm and the value of Rth/Ro of from 2.6 to 5.0; and also discloses, as an example of the cellulose derivative which can be used for preparing the film, cellulose acylate propionate.
Recently, more improvement in displaying-qualities has been required in terms of viewing-angle characteristics and coloration in the black state, and achieving these, more reducing the ratio, Rth/Re, of the cellulose acylate film to be used in a liquid crystal display device has been required.